Open question: Can the selected BMEditMeshes have different selectmodes? The current assumption is, that the selectmode is the same for all selected meshes in editmode.
If they can in fact be different, why? Isn't the mode something global in 2.8?

Oleg are those two different objects? If so I don't think it will work. You also cannot create edges or faces between different objects.

We could have a better error message here though.

Yes, this is different objects (Multi-Object-Mode edit). I thought it should automatically join those objects together when use MESH_OT_bridge_edge_loops... Then maybe this tool simply should not be available when selecting components from two different objects?

You might want to bridge faces on several objects at a time. This works now.

Blender does not automatically join meshes if you try and do these kinds of operations. While it could be useful, it's also dangerous. You could accidentally completely screw up your scene or rig if we did this.

We should display better error messages if you try this, though, or present a popup that asks the user if they would like to join them.

Please don't use this task as a place for bug reports. In the future open a new report. Although this is not a crash, it is a feature considered done and then a valid bug (if the reporter went through the trouble of checking if the operator is marked as done). That said, William's explanation is point on, and the tool is behaving as designed.